Communications through geostationary satellites with mobile vehicles on or around the earth's surface is best done if the antenna on the vehicle is directional in that a received signal will have higher gain than if a non-directional antenna is used. However, the directional antenna must be continuously pointed toward the satellite as the vehicle moves and changes directions. It is also advantageous if the directional antenna continues to point in the direction of the satellite even if the signal path between the vehicle and the satellite is interrupted which occurs when the vehicle passes through a tunnel or under an overpass or otherwise is in the vicinity of an interfering building or other objects which tend to impede or otherwise adversely affect the received signal strength.
Prior known means to steer mobile antennas toward satellites have used continuous measurements of a signal received from the satellite to determine the direction to the satellite and to couple this continuously received signal with a servo mechanism which maintains pointing of the antenna toward the satellite. Signal measurement means for this purpose have included monopulse and antenna beam lobing techniques. Such means are usually complex, expensive and require a new acquisition of the signal after each period of signal interruption for antenna steering purposes. This known means may also require signal sampling rates that are within the bandwidth of the information contained in the received communication signal and thus may adversely affect the quality of the communication signals. These prior art means also have been known to employ a gyroscope or compass or other similar means to provide a direction reference that then is used to maintain pointing during periods of signal loss or degradation. See for example U.S. Pat. No. 4,630,056 issued Dec. 16, 1986 for a "Control System for Antenna of Receiving Equipment Installed on Moving Body". Gyroscopes suitable for application of this nature are generally too costly for many of the potential mobile satellite services. Magnetic compasses are subject to unacceptable directional error due to mechanical instabilities variable declinations of the earth's magnetic field and to deviation and variation due to magnetic materials in the vicinity of the magnetic compass.
In order to overcome a number of the known problems encountered with prior art directional antenna steering systems including their relatively high cost, the present invention was devised.